The specific aims of this protocols are to test (a) whether clopidogrel plus aspirin prevents more thrombus formation than aspirin alone and, (b) measure the antithrombotic response of clopidogrel at 2 hours after an oral dose of 300 mg (load) vs 75 mg (no-load) in comparison with one week of treatment in patients with chronic coronary or peripheral arterial disease and on chronic aspirin 325 mg per day.